Stage 1—Transition to sleep
Lasts about five to ten minutes. During this stage you may fall asleep but think you are still awake. Car accidents caused by falling asleep usually happen during this stage. The eyes move slowly, muscle activity slows down, and you easily wake up. Your muscles may twitch and you may experience the sensation of falling.

Stage 2— Light sleep
The first stage of real sleep lasts about twenty-five minutes. The heart rate slows down, and rebuilding begins to take place in the body and brain.

Stage 3—Deep sleep
It is difficult to wake up during this stage, and if you do, you will feel groggy and disoriented for several minutes. Your body secretes many hormones into the blood stream including human growth hormone, which experts believe is related to the repair process that takes place while you sleep.

Stage 4—More intense deep sleep
This is the deepest stage of sleep. Brain waves (called Delta waves) are very slow, and blood flows to the muscles to help restore physical activity. Many sleep experts combine stages three and four.

REM sleep—Dream sleep
Seventy to ninety minutes after falling asleep, breathing becomes more rapid, eyes move rapidly, muscles feel paralyzed, and brainwaves increase to the same level as awake. Heart rate increases and blood pressure rises. Most dreams occur during the REM stage. If you are awakened from this stage, you will most likely remember your dreams. Most people experience three to five intervals of REM sleep per night.

The Ninety-Minute Rule for Optimal Sleep

If you are continually struggling to get out of bed when your alarm goes off, and you feel as if you are waking from the middle of deep sleep, try the ninety- minute trick. Resetting your wake up time in increments of ninety minutes will make it easier for your brain to wake up. So, if you usually go to bed at 10:30 pm, set your clock to rise at 6:00 am instead of 6:30 or 7:00 (five ninety-minute increments for a total of seven and a half hours of sleep). You will feel more refreshed as your body and brain are already close to waking up, and you are past the level of deep sleep. (Adapted from Your Guide to Healthy Sleep, by the National Institutes of Health).

Here’s a free Sleep Calculator to help you figure out what time to go to bed or what time to wake up.